Monday, 14 July 2008

'Crewe' All Electric System, LNWR

You may have noticed my admiration for all things connected with the London & North Western Railway, particularly in the signalling area. But I was slow to discover how early the L&NWR was in developing power-operated signalling. This was called the 'Crewe' All Electric system and it was aimed at complex and busy installations. This brief outline is based on displays at Crewe Heritage Centre with grateful acknowledgment.

The miniature lever signalling frame developed for the All Electric system was patented by F. W. Webb (Chief Mechanical Engineer) and A. M. Thompson (Signal Engineer). The equipment was built in the L&NWR workshops at Crewe. In addition to conventional mechanical interlocking between the levers, electrical locks could be provided at mid-stroke, so that lever movement could only be completed once points had operated to the correct position. Movement of the lever operated electrical switches which used large carbon blocks. These switches could be arranged to be closed in the unoperated (Normal), operated (Reverse) and Midstroke ('C') positions. Where space was limited, all this equipment could be mounted within the frame on the operating floor of the box. The photograph shows the 'two tier' lever frame at Gresty Lane No. 1 in 1899. This type of equipment was installed at various signal boxes around Crewe. Similar installations were made at Euston No. 3 and Camden No. 1 in 1905.

In 1908/1909, Manchester London Road was re-signalled with an improved version of the 'Crewe' All Electric system installed in three signal boxes. The picture shows the No. 2 box in 1957. In addition to the miniature lever frame, notice the L&NWR Block Instruments and L&NWR Train Describers still in use. The L&NWR built things to last! These boxes survived until electrification of the line.

In the 'Crewe' All Electric system, points were operated by motors (although early installations may have used solenoids). This picture shows a pilot point installation within Crewe Works. The large cast cover on the left is where the point motor was installed: electrical detection was under the smaller cover on the right. Both 110 volts d.c. and 220 volts d.c. were used at different installations. Around 15 Amps was required to drive the motor at 220 volt d.c.

Signals for the 'Crewe' All Electric system were lower-quadrant semaphore, using many of the standard parts for mechanically-operated signals. The sketch is derived from a fairly well-known drawing (which I'm sure must have been produced by J. K. Nelson). The solenoid to operate the arm (visible in 'cutaway' on the sketch) was mounted on the post a few feet below the arm. Energising the solenoid pulled the left end of the weight bar up so that the push rod turned the arm to the 'off' position. The balance weight at the right end of the weight bar was arranged to assist the solenoid in pushing the arm off - this, of course, is opposite to normal practice with mechanical wire-operated semaphores. At 220 volt d.c. operation, around 8 Amps was required to move the arm to 'off'. Once the arm was 'off', an electrical contact operated to reduce the solenoid current to about 2 Amps, sufficient to hold the arm 'off' but significantly economising on the electrical power required.

This picture (apologies for poor quality) shows a gantry of solenoid-operated semaphores at Euston Carriage sidings. Note the vitreous-enamelled 'VIROL' advertisement on the retaining wall. As far as I can make out, the locomotive in the left background is one of the ubiquitous 0-6-0T 'Standard Shunt', frequently used on empty stock and banking duties at Euston.

The first signal boxes I saw which used the Crewe 'All Electric' system were Crewe Station 'A' (halfway along the old platform 1) and Crewe Station 'B' (halfway along the old platform 2, now platform 12). Crewe Station 'A' box survives (heavily modified by both the L.M.S. and British Railways), now triumphantly rebuilt in Crewe Heritage Centre. There are more pictures of the rebuilt box here.